NOx catalysts purifying NOx (nitrogen oxide) contained in exhaust gas are commonly known as exhaust purification apparatuses located in exhaust systems of internal combustion engines such as diesel engines or lean burn gasoline engines. Various types of NOx catalysts are known, and an NOx catalyst of storage reduction type (NSR: NOx Storage Reduction) is well known to absorb and remove NOx in exhaust gas. The NOx catalyst of storage reduction type exerts an NOx absorbing and releasing effect so as to absorb NOx in supplied exhaust gas when the exhaust gas has an air-fuel ratio leaner than a predetermined value (typically, a theoretical air-fuel ratio) (that is, the exhaust gas is in an oxygen excess atmosphere), while releasing the absorbed NOx to reduce NOx to N2 when the exhaust gas has an air-fuel ratio richer than the predetermined value (that is, the exhaust gas is in an oxygen poor atmosphere).
Once the NOx catalyst of storage reduction type is saturated with NOx, that is, the NOx catalyst is full of NOx, as a result of absorption, the NOx catalyst can no longer absorb NOx. Thus, at appropriate time intervals, the NOx catalyst is supplied with a reducing agent and thus set in an oxygen poor atmosphere. The absorbed NOx is thus released from the NOx catalyst, which thus recovers the NOx absorbing capability. This is called NOx recovery.
For example, to determine timings for the beginning and end of the NOx recovery, an NOx sensor is provided downstream of the NOx catalyst in order to detect the concentration of NOx in the exhaust gas. For example, when the NOx catalyst becomes full of NOx as a result of absorption, NOx starts to leak downstream of the catalyst. Thus, the NOx recovery may be started when the NOx sensor detects the leaking NOx. Furthermore, when the NOx concentration detected by the NOx sensor during the NOx recovery decreases sufficiently, all of the absorbed NOx is determined to have been released. Thus, the NOx recovery may be ended.
For example, for engines mounted in vehicles, laws in many countries require that the abnormality of the catalyst or the sensor be detected in a vehicle mounted condition (onboard) in order to prevent possible traveling in a deteriorated exhaust gas condition. Relatively many techniques are available for detecting the abnormality of the catalyst. However, at present, no effective technique is available for detecting the abnormality of the NOx sensor provided downstream of the NOx catalyst as described above. In particular, emission restrictions have been tightened, and there has been a demand for proper detection of not only defects such as an open circuit but also the rationality of sensor outputs for degradation or the like. Thus, drastic measures for dealing with this are required.
For the abnormality diagnosis of the NOx sensor, a method is possible in which a plurality of NOx sensors are provided at the same position so that detected values from the sensors can be relatively compared with one another or in which the NOx sensor is removed and checked using a fixed analyzer. However, the former method increases costs, and the latter method prevents onboard diagnoses.
Japanese Patent Application Laid-Open No. 2003-120399 discloses an abnormality detection apparatus for an NOx sensor provided downstream of an NOx absorber. The NOx concentration of exhaust gas reaching the NOx sensor is forcibly varied. If variation in NOx sensor output value deviates from possible variation during the normal condition of the sensor, the NOx sensor is determined to be abnormal.
However, the exhaust gas reaching the NOx sensor has passed through the NOx absorber. Thus, the NOx concentration of the exhaust gas corresponds to one obtained after NOx has been absorbed by the NOx absorber. That is, the impact of the NOx absorber located before the sensor is reflected in the output value from the NOx sensor. This may reduce the accuracy of the abnormality diagnosis of the NOx sensor.